The present invention relates to audio processing and modulation systems and, more particularly, to an improved signal processing system intended for maximizing the efficiency of line system transmission or transferring especially of audio frequency signals, as in telephony, for providing high modulation levels and for enhancing signal intelligibility and clarity while avoiding loss of audio dynamics.
In both long and short distance line signal transmission and reception of modulated signals, such as utilized in telephone and other transmission line systems utilizing any combination of various signal transmission modes, such as twisted pair, RF link, laser, fiber optics, and so forth, a major problem has always been to obtain a high level of dynamic amplitude while not only containing the full spectrum of speech harmonics but also keeping the audio bandwidth of the transmitted line signals as narrow as possible. Many modes of line transmission and types of audio or other signal modulation have been used where these matters are of great concern. Principal forms of modulation presently in use are AM, SSB, and FM. AM and FM utilize a constant carrier principal while SSB has a direct audio into power out relationship and is not a constant carrier mode of transmission when the carrier is suppressed, as is quite common.
Frequency Modulation (FM) PA0 Amplitude Modulation (AM) PA0 Single Side Band (SSB) PA0 Characteristics of Human Speech with Reference to Radio Transmission PA0 Line Systems
In constant carrier FM mode, the dynamic content of the signal and the audio bandwidth of the signal are directly proportional to the amount of deviation allowed to be imposed on the carrier. E.g., in narrow band FM transmission line systems, an audio bandwidth of 3 kcs. may be imposed. As a result, dynamic harmonics of voice characteristics are restricted and lost.
In the constant carrier mode of amplitude modulation, dynamics are expressed in a direct relationship with the amplitude of the constant carrier and audio frequency bandwidth being theorectically limited only by the line frequency bandwidth available. In commercial line communication systems utilizing the amplitude modulation mode of transmission, the audio bandwidth usually is restricted to eliminate spurious AM modulated pulses resulting from ignition and other electrical noise being induced on the line. An AM mode now commonly used takes the form of pulse amplitude modulation (PAM), as in teletype and data transmission systems.
The more recent mode of SSB modulation is by far the most efficient form of transmitting an audio modulated high frequency line signal. The major drawback of this mode of modulation is the extremely restricted audio bandwidth imposed resulting in extreme losses of dynamic intelligibility. However, these losses are often traded off against the high efficiency of single side band line systems.
Relative to radio transmission, attributes of human speech of concern are dynamic amplitude and harmonic relationship. The latter is extremely important in identification intelligibility.
Dynamic amplitude can be defined as the varying level of audio received by a modulation stage in any mode of modulation. The human voice is made up by a complex structure of harmonics, the main bands of harmonics falling within a 3 kHz bandwidth. A speech band-pass frequency range commonly selected is 300 Hz to 3000 Hz, and all other harmonics are generally suppressed. However, these out-of-band harmonics define voice character and, thus, intelligibility. But the suppressed harmonics fall in such a wide spectrum that if the entire speech harmonic make-up were to be transmitted, a line transmission bandwidth of some 15 kHz would be required. With modern narrow band voice line transmission systems, this would become impossible.
Consequently, speech processing has been utilized heretofore. For example, the use of band-pass frequency filters in speech line transmission systems is very common. Although speech band-pass frequencies often vary, the pass band rarely exceeds 3 kHz. This type of processing is used chiefly with narrow band audio line transmission systems. It has the advantages of being not only simple and economical to provide but also offering the expedient of limiting the power band of speech to a 3 kHz window that can be readily utilized in audio line transmission systems.
When electrically processed, signals representing human speech inevitably vary greatly in amplitude. The audio content of the transmitted signal thus varies accordingly. Yet the received audio level can only be taken as an average between the maximum and minimum dynamic amplitudes transmitted. One way of increasing the average received audio level is to use a speech compression system between the audio sound and the modulation system, resulting in a higher average level of modulation and effectively increasing the transmitted audio power. However, in increasing the lower level sequences to effectively increase the average audio content of the transmitted signal, all speech dynamics have to be sacrificed. Thus, increase in the transmitted or received audio level, after compression, must be traded off against speech intelligibility. Audio compression has been best utilized in teletype and data transmissions systems in which single frequency pulses are transmitted and dynamics are not of concern.
Circuitry providing gain control for high and low audio bands and subsequent compression of a mixed audio signal is described in Bloy U.S. patent application Ser. No. 282,051, entitled "Complete Audio Processing System", which is a continuation of application Ser. No. 59,394.
In addition to filtering and compression techniques, various other types of processing have been used in commercial line transmission systems. Where the speech input to the line modulator is not considered adequate in dynamic amplitude, simple pre-amplifier devices are often used.
Owing to the fact many present day line systems not only carry audio information in the form of speech but also carry digital and pulse amplitude modulated carriers, a system for interfacing audio processing circuitry with line systems should be designed to handle these various modes of line transferred information. In multiplex carrier systems, frequency modulation is also used. To accommodate these functions, interfacing should be able to prove a system contains demodulation and/or modulation for AM, FM, CW and SSB modes of transmission over a communication link.
An object of the invention is to provide a system for maximizing the efficiency of transfer of modulated signals in general and especially audio frequency energy, and particularly such a system utilizing transmission line systems.
A further object of the invention is to provide such a system useful with modulation systems.
A further object of the invention is to provide such a system for providing transfer of audio frequency signals, especially over transmission lines, in such a way that high average modulation power is attainable.
A still further object of the invention is to provide such a system for providing transfer, especially over transmission lines, of signals particularly those of audio frequency, to enhance signal intelligibility and clarity while preventing loss of dynamics.
Another object of the invention is to provide such a system which allows the processing of signals demodulated from received line transmitted signals in order to retrieve signals, particularly audio signals, with effectively high signal-to-noise ratios even where there is high noise level associated with the received line transmitted signals.
A further object of the invention is to provide such a system which can be utilized for processing of various types of signals, e.g., voice, tones, data, etc., prior to their use in modulating a line transmitted signal, as well as processing demodulated audio frequency signals upon reception of a line transmitted signal.
An additional object of the invention is the provision of such a system which is useful with various modulation systems and modulation stages to achieve maximum carrier utilization, with result in increase in effective line transmitted power and clarity.
Another object of the invention is the provision of such a system which processes audio frequency signals in such a way as to reconstitute wide frequency spectrum dynamics associated with voice and other audio frequency signals used, inter alia, for modulation of transmission lines, and which does so by recovery and amplification to audible levels of harmonics otherwise suppressed or filtered.
A related further object of the invention is the provision of such a system which permits transmission of audio signals with high average modulation levels, approaching 100% average modulation, but without customary dramatic loss of audio dynamics and intelligibility.
An object of the invention also is the provision of such a system which can be utilized in the fields of line transmission not only of speech but also high speed data, teletype, facsimile, and other modes of data communication, and when so utilized in data communication, may be advantageously part of a data communication link for reducing data drop-out.
It is also an object of the invention to provide such a system which not only is relatively simple and utilizes integrated, compact circuit components but which also is conducive to simple operation, having a minimum of useable controls that once set to the application at hand need no further adjustment.
A related further object of the invention is the provision of such a system utilizing various visual indicators for keeping the user constantly informed and aware of the extent to which signals are properly processed through the system.
An additional object of the invention is the provision of such a system which accepts substantially any low level signal within the audio frequency spectrum while according to the user the options of utilizing and emphasizing various components of the audio spectrum, and of selectively filtering and/or amplifying the input and/or output signals.
Another object of the invention is the provision of such a system in which the output of the audio signals processed through the system can be preset in amplitude to be fed to any modulation stage currently used in conventional line communication systems.
A further object of the invention is the provision of such a system which is especially of advantage in connection with narrow band VHF line transmission, making possible extraordinarily narrow band transmission containing full dynamic characteristics of audio frequency signals so processed while keeping a very high character level of modulation.
Among other objects of the invention may be noted the provision of such a system which can be utilized in connection with AM, FM, SSB, PAM, FSK (frequency shift keying), PSK (phase shift keying) and tone activated TTY line transmissions, which also can be utilized in public address application and music amplification systems.
A further object of the invention is the provision of such a system which provides interfacing with telephone lines providing impedance matching and automatic selective direction control for transmission and receiving functions.
Finally, among still other objects of the invention may be noted the provision of such a system which allows individual tailoring of harmonic response curves depending upon natural frequencies of voice signals; which allows continuous audio frequency gain control; which achieves linear tracking during audio processing; which operates to eliminate or greatly reduce third harmonic distortion; which operates to reconstitute out-of-band dynamics for allowing them to be transmitted on a narrow band signal; which makes use of solid state integrated circuit technology; which is essentially uncomplicated as well as being simple to use and maintain; which can advantageously be operated from a low voltage or battery power supplies; and which exhibits low power consumption and inherent high efficiency during operation.
Other objects and features will be in part apparent and in part pointed out hereinbelow.
Briefly, a signal transfer and processing system of the invention includes interface circuitry and processing circuitry. The processing circuitry includes a signal input for receiving signals (such as audio frequency, telephone, digital, encypted, or data signals, etc.) to be processed. These signals are supplied to a bandpass input filter and, thus filtered, to a primary active frequency control. The output of the latter drives a primary voltage compressor which selectively limits signal dynamics to a predetermined window, being controllably preset and driven with different bands of amplitude determined by the primary frequency control to maximum compression levels indicated by a visual indicator. The output of the compressor is presented to a secondary active frequency control which drives a secondary dynamic compressor with different audio bands. A second signal indicator indicates maximum compression levels of the latter. An automatic gain control tied to the latter compressor supplies feedback to the primary voltage compressor and also feeds a third visual indicator which displays compressed output voltage as well as average peak dynamic compression. The processed output of the secondary compressor is provided to a bandpass output filter and, sharply attenuated by the latter, is delivered for further use. A high level output stage is also selectively utilized. The AGC feedback signal to the primary compressor limits primary compression as a time-delayed function of increase in the level of the secondary compressor output. The primary and secondary compressors may be regarded as respective first and second dynamic control means, the primary and secondary active frequency controls as respective first and second tonal control means, since both compressors and both active frequency controls are configured for permitting selective controlling of their respective functions. The bandpass input and output filters each may be selectively switched in or out of the signal processing path. The third visual indicator may be a meter having a moving coil-type movement to provide an averaging display. The interface circuitry provides interconnection of said processing circuitry with a transmission line for causing signals transmitted over the transmission line to be automatically directed through the processing circuitry thereby to produce processing of audio frequency signals transmitted over the transmission line. The interface circuitry includes switching circuits for providing interconnection with telephone lines, for example, and for selectively switching into the system demodulators and modulators for AM, FM, CW and SSB modes of communication.